1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a CMOS image sensor suitable for improving light-condensing efficiency.
2. Discussion of the Related Art
In general, an image sensor is a semiconductor device for converting an optical image to electric signals, which is largely classified into a charge coupled device (herein after, referred to as CCD) and a CMOS image sensor.
In case of the CCD, respective MOS (metal-oxide-silicon) capacitors are adjacent to one another, wherein electric charge carriers are stored in and are transferred to the capacitors. Meanwhile, the CMOS image sensor uses a CMOS technology of using a control circuit and a signal processing circuit as peripheral circuits. That is, in case of the CMOS image sensor, MOS transistors are provided in correspondence with pixels, and outputs are detected in sequence, thereby applying a switching method.
The CCD has the disadvantageous characteristics of the high power consumption and the complicated fabrication process generated by a large number of masks. In the CCD, it is impossible to provide the signal processing circuit within a CCD chip, whereby it cannot be provided as one chip. In order to overcome these problems, the CMOS image sensor using a sub-micron CMOS technology has been researched and developed.
The various pixels may be used for the CMOS image sensor. Among them, a 3-T structure pixel and a 4-T structure pixel are most generally used for the CMOS image sensor. In more detail, the 3-T structure pixel is comprised of three transistors and one photodiode, and the 4-T structure pixel is comprised of four transistors and one photodiode.
FIG. 1 shows a circuit view of a unit pixel in a general 4-T CMOS image sensor.
As shown in FIG. 1, the unit pixel of the 4-T CMOS image sensor is provided with a photodiode PD for sensing light, and four NMOS transistors (Tx, Rx, Dx, Sx).
Among the four NMOS transistors, the transfer transistor Tx transfers optical charges generated in the photodiode PD to a floating sensing node, and the reset transistor Rx discharges the optical charges stored in the floating sensing node to detect signals. Also, the drive transistor Dx functions as a source follower, and the select transistor Sx is provided for switching and addressing.
In addition, ‘DC gate’ is a load transistor for maintaining a constant current by applying a constant voltage for a gate potential of the transistor, ‘VDD’ is a driving voltage, ‘VSS’ is a ground voltage, and ‘output’ is an output voltage of the unit pixel.
Hereinafter, a related art CMOS image sensor will be described with reference to the accompanying drawings.
FIG. 2 shows a schematic view of a CMOS image sensor according to the related art. In the related art CMOS image sensor, as shown in FIG. 2, a semiconductor substrate 11 is defined as a field area and an active area by STI films 12 formed by a trench isolation technology. Also, a photodiode PD is formed in the semiconductor substrate 11 of the active area between the STI films 12, wherein the photodiode PD receives the incident light and stores the electric charges. Then, metal circuits M1, M2, and M3 are provided to form the unit pixel, and insulating interlayers 13, 14, 15, and 16 are provided to insulate the metal circuits M1, M2, and M3 from one another.
As described above, the unit pixel of the CMOS image sensor is provided with the photodiode PD and the plurality of transistors for the signal processing. In this state, the metal circuits M1, M2, and M3 are provided to form the unit pixel, wherein the metal circuits M1, M2, and M3 are positioned not to prevent the incident light.
In addition, a protective layer 17 is formed on the insulating interlayer 16, so as to protect the photodiode PD and the metal circuits M1, M2, and M3 from external moistures and scratches. Then, a planarization layer 18 is formed on the protective layer 17, to solve the problem of step coverage, and to improve adhesion.
On the planarization layer 18, a color filter layer 19 is formed, wherein the color filter layer 19 is formed of dyeable photoresist of red, green, and blue above the photodiodes PD. After that, an overcoat layer 20 is formed on the color filter layer 19, wherein the overcoat layer 20 is provided to overcome the problem of step coverage, to obtain uniformity of micro-lens, and to control a focal distance. Then, the dome-shaped micro-lens 21 is provided on the overcoat layer 20, so as to condense the light to the color filter array.
However, the related art CMOS image sensor has the following disadvantages.
With the recent trend toward high-resolution and high-quality image sensors, there is requirement for decreasing the size of device. As a result, it is necessary to decrease a design rule, whereby the size of unit pixel in the CMOS image sensor decreases gradually.
Accordingly, as compared with the size of photodiode PD, upper structures, for example, the insulating interlayers and metal lines provided above the photodiode PD, increase, so that the distance from the micro-lens 21 to the photodiode PD increases. In this state, it is difficult to fabricate the micro-lens for controlling the focus of the photodiode PD.
Also, as compared with the focal distance of the micro-lens, the photodiode PD is distant from the micro-lens. In this state, the light refracted by the micro-lens may not be received to the photodiode PD.